bitbake.conf: Handle S and B separately for debug mapping

bitbake.conf: Handle S and B separately for debug mapping

We don't really need to keep S and B separate for debug source purposes
and there shouldn't be source references in WORKDIR that isn't S and B
either.

Separating these out simplifies the shared-work directory handling for
gcc and should also help fix external source usage. Therefore handle
S and B in DEBUG_PREFIX_MAP separately and clean up other code.

Indentation is reduced here as it is introduced on every compiler
commandline so minimising it is helpful.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>